In recent years, data communications have been under shift to those via optical fibers and accordingly, the data transmission speed has been enhanced more dramatically than before.
It is under consideration to conduct communications at a transmission speed of 160 Gbit/s which is much higher than the current transmission speed, or greater, using ultra-short pulses in such data communications via optical fibers in near future.
Problems of crosstalk and transmission errors always accompany data communications, but as the data transmission speed is enhanced, the width of each optical pulse and the interval between optical pulses in tandem with each other spontaneously decrease, and therefore the problem becomes very important.
The speed at which light travels through a material is determined by the refractive index of the material, and the speed of light decreases as the refractive index increases. In materials such as glass, semiconductors and optical crystals, the refractive index varies with the frequency of light (wavelength in air), and therefore the speed of light depends on the wavelength. It is known that due to this wavelength dependence of the refractive index, the waveform of an optical pulse is distorted and the time length of the pulse increases while the optical pulse travels through a material. A characteristic such that the speed of light varies depending on the wavelength of light in this way is hereinafter referred to as chromatic dispersion or merely dispersion.
As described above, the waveform of the optical pulse is distorted and the time length of the optical pulse increases while the optical pulse travels through optical fibers, but there arises no significant problem because the time length of the optical pulse is large at a conventional transmission speed. However, if the data transmission speed increases, optical pulses in tandem with each other interfere with each other, and so on, to cause crosstalk and transmission errors. Therefore, data communication at a higher speed cannot be achieved by merely enhancing the transmission speed with a current technique.
For countering this problem, an attempt has been already made to compensate chromatic dispersion using, for example, a photonic crystal.
The photonic crystal has a structure in which two materials having different refractive indexes are periodically arranged, and part of this arrangement is made defective to form a defect waveguide (continuous defect part), whereby only light having a specific frequency passes, and a waveguide mode giving specific chromatic dispersion to the light occurs. By using this waveguide mode, chromatic dispersion of an optical fiber transmission line is compensated (see, for example, Kazuhiko Hosomi, Toshio Katsuyama, “Light Propagation Characteristics of Photonic Crystal Coupled Defect waveguide (2)”, “Proceedings of 63rd Academic Meeting of Japan Society of Applied Physics, Vol. 3”, Japan Society of Applied Physics, Sep. 24, 2002, p. 917).
In addition, a technique using an optical fiber diffraction grating as a dispersion compensation element is implemented as a structure similar to that of the photonic crystal. This is an attempt to compensate chromatic dispersion in a broad spectral band using a chirp optical fiber diffraction grating in which the period of the diffraction grating is changed along the length of the optical fiber (see, for example, Akira Suzuki, Shinichi Wakabayashi, “Technique for Dispersion Compensation of Short Pulse”, “Optronics”, Optronics Co., Ltd., 2002, vol. 21, No. 4, p. 161-165).
For dispersion compensation for optical pulses generated by an ultra-short pulse laser for physicochemistry, techniques using a prism pair or a diffraction grating pair have come into widespread use. These techniques are intended for compensating mainly positive chromatic dispersion (see, for example, J-C Diels, W Rudolph, “Ultrashort Laser pulse Phenomena”, USA, Academic Press, 1996, p. 43-99).
However, further enhancement of the transmission speed cannot be sufficiently accommodated by simply using the conventional chromatic dispersion compensation technique described above.
That is, chromatic dispersion results from the dependency of a phase of an optical pulse on the wavelength (or frequency, hereinafter referred to simply as wavelength) as described above. Generally, the phase of an optical wave is expressed as a polynomial expression expanded with terms different in order (power index) of the wavelength with a certain wavelength at the center. It is known that the coefficient of the second-order term corresponds to the lowest-order chromatic dispersion, and as coefficients of terms of subsequent orders, coefficients of third-order, fourth-order and fifth-order terms follow (see, for example, Keisuke Ogawa, “Measurement of Ultra-Short Optical Pulse”, “Ultrahigh Speed Optoelectronics Technique Handbook”, Sipec Corporation, Jan. 31, 2003, Chapter 2, 2.4).
For the current optical pulse transmission speed, compensation of chromatic dispersion for the second-order term is enough, but as the transmission speed is enhanced, the time length of the optical pulse for use in data transmission becomes shorter, and in inverse proportion thereto, the spectral bandwidth of the optical pulse increases. Thus, as the transmission speed is enhanced, distortion of the waveform of the optical pulse cannot be eliminated unless wavelength compensation coefficients of up to higher orders are compensated over a broader spectral band.
In the conventional chromatic dispersion compensation technique using a photonic crystal or optical fiber grating, chromatic dispersion can be compensated for each order such as second, third or fourth order, however, chromatic dispersion cannot be compensated for multiple orders. Chromatic dispersion compensation meeting ultrahigh-speed large-capacity optical communication using a broad spectral band cannot be achieved.
An ultrahigh-speed large-capacity optical fiber transmission line is designed so as to have an optimum optical pulse transmission characteristic in itself. That is, it is configured such that chromatic dispersion is zero as the entire transmission line.
However, for example, an optical fiber transmission line laid on the sea bottom or the like may depart from conditions under which the optical fiber transmission line is optimized by influences of temperature, atmospheric pressure, vibrations and the like. In such a situation, the chromatic dispersion in the optical fiber transmission line constantly changes between positive and negative signs.
In contrast to this, in the conventional technique, it is difficult to make the sign of chromatic dispersion compensation variable independently of the absolute value of chromatic dispersion when the chromatic dispersion changes between positive and negative signs. This means that it is difficult to cope with a situation in which the chromatic dispersion value constantly changes around zero between positive and negative signs.
The “positive” chromatic dispersion indicates that the speed of light increases as the wavelength increases, and the “negative” chromatic dispersion indicates that the speed of light decreases as the wavelength increases.
The present invention has been made based on such technical problems, and its object is to provide a dispersion compensation element, a dispersion compensation system and the like which can enhance the transmission speed of optical pulses.